The invention relates to a wavelength-stabilized laser configuration containing a semiconductor laser and a regulating device for regulating a laser beam.
It is already known to use laser modules based on a semiconductor laser in optical transmissions systems, which laser modules are operated using a wavelength division multiplex (WDM) method in order to maintain a high transmission capacity. In order to be able to transmit as many channels as possible, the laser modules have to meet high requirements with regard to their wavelength""s stability.
Furthermore, it is already known to equip high-rate laser modules of this type with an optical isolator. The optical isolator has the function of suppressing undesirable perturbations of the semiconductor laser due to reflected laser light.
A wavelength-stabilized laser configuration is described in Published, Non-Prosecuted German Patent Application DE 197 12 845.9 A, which was not published before the priority date. For the purpose of wavelength stabilization, two partial beams are coupled out of the laser light pencil and detected by respective detectors. Beam splitters or mirrors are used for coupling out the partial beams. At least one of the partial beams passes through an optical filter with wavelength-dependent transmission, with the result that the corresponding detected signal is wavelength-dependent. By comparing the wavelength-dependent signal with the wavelength-independent signal of the other detector, the instantaneous wavelength of the semiconductor laser is determined and a deviation with respect to a predetermined desired value is determined. The laser wavelength is then set to the desired wavelength by temperature regulations of the semiconductor laser.
It is accordingly an object of the Invention to provide a wavelength-stabilized laser configuration which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which a simple and cost-effective structure of a laser configuration with a semiconductor laser is provided, and the laser configuration is suitable for high transmission capacities and, in particular, for wavelength division multiplexing.
With the foregoing and other objects in view there is provided, in accordance with the invention, a wavelength-stabilized laser configuration, containing:
a semiconductor laser outputting a laser beam having a laser power, a laser beam path, and a laser light;
a regulating device, including:
a first optical detector receiving a first partial beam derived from the laser beam and outputting a first optical detector output signal;
an optical filter;
a second optical detector disposed downstream of the optical filter and receiving a second partial beam derived from the laser beam and filtered by the optical filter, the second optical detector outputting a second optical detector output signal;
a signal processing device receiving the first optical detector output signal and the second optical detector output signal and determining from the first optical detector output signal and the second optical detector output signal an actual value signal being representative of an instantaneous laser wavelength xcex being independent of the laser power;
a comparison device connected to the signal processing device for comparing the actual value signal with a predetermined desired value signal specifying a desired wavelength xcexc; and
a drive device connected to the comparison device, the drive device generating a control signal setting an operating parameter of the semiconductor laser, the operating parameter influencing a wavelength to be stabilized such that the actual value signal substantially corresponds to the predetermined desired value signal; and
an optical isolator disposed in the laser beam path and having a first polarizer, a device for rotating a plane of polarization of the laser light and connected downstream of the first polarizer, and a second polarizer connected downstream of the device for rotating the plane of polarization of the laser light, at least one of the first polarizer and the second polarizer being used as a beam splitter for coupling out one of the first partial beam and the second partial beam.
The laser configuration according to the invention has both wavelength stabilization and an optical isolator and, therefore, is especially suitable for the transmission of high data rates in WDM operations. According to the invention, at least one of the partial beams required for the wavelength stabilization is in this case coupled out by a polarizer of the optical isolator. The polarizer thus fulfills two functions at the same time, namely that of a polarizer in the optical isolator and that of a beam splitter in the wavelength stabilization circuit. As a result, a beam splitter as an additional component is obviated and a simple and compact overall structure is obtained.
It is preferable for (at least) the second polarizer to be used as a beam splitter and to be used for coupling out the second partial beam. The fact that the optical isolator suppresses reflections from the filter and from the second detector and, consequently, prevents these from having a perturbing effect on the semiconductor laser has a favorable effect in this case.
Furthermore, it is expedient for the first polarizer (also) to be used as a beam splitter for coupling out the first partial beam. In this way, the integration level and the compactness of the configuration according to the invention are further promoted by the obviation of a further beam splitter.
A preferred refinement of the invention is characterized in that the filter is a cut-off filter or a bandpass filter. If a bandpass filter is used, it is operated at one of the cut-off wavelengths of the bandpass filter.
A further preferred refinement of the invention is characterized in that use is made of a further filter for filtering the first partial beam that is guided to the first detector. In this case, it is advantageous that both filters are operated with linearly polarized light. This reduces the requirements to be made of the filters and makes it possible to use cost-effective filters.
If two filters are used, it is preferred to arrange a xcex/2 plate with suitable orientation of its optical axis in the beam path of the first partial beam between the first polarizer and the further filter. The xcex/2 plate serves to suppress reflections occurring at the further filter (so-called xe2x80x9cquasi-isolatorxe2x80x9d).
It is preferable for the further filter to have a filter curve that is shifted relative to the filter curve of the filter and in the opposite direction with regard to the desired wavelength xcexc. By evaluating the detector signals in a suitable manner, it is possible in this case to obtain a wavelength sensitivity which, given the same filter gradient is twice as high as that obtained when just one filter is used.
If two filters are used, it is furthermore advantageous if the filter curve of the filter and the filter curve of the further filter have a point of intersection at the desired wavelength xcex0. In this case, the wavelength of the semiconductor laser can be adjusted by zero regulation to the desired wavelength xcex0.
A further preferred refinement of the invention is characterized in that the filter and/or the further filter are configuration in a manner that allows them or it to pivot. The pivotable configuration of the filter makes it possible to vary and adjust the center wavelength xcexM of the filter by tilting the filter relative to the axis of the incident partial beam. Therefore, a whole range of different desired wavelengths can be covered with the same filter, as a result of which changing conditions of use can also be controlled without the need for exchanging the filter. Furthermore, a simple possibility of adjustment is afforded, which allows the tolerance requirements that are to be made of the filter to be reduced with regard to adhering to the center wavelength xcexM.
Instead of the filters, the second polarizer and/or the first polarizer may also be disposed in a manner allowing them or it to pivot, as a result of which the same effect is obtained.
A particularly compact configuration is achieved if the second polarizer and the filter are disposed in a mutually positionally fixed relationship in a common, pivotable unit.
The device for rotating the plane of polarization is advantageously configured as a Faraday rotator. In this case, a preferred refinement of the invention is characterized in that the Faraday rotator itself is permanently magnetized. Faraday rotators of this type need not be equipped with an external permanent magnet, which can impede, or make more difficult, the lateral coupling out of the partial beam or partial beams to be coupled out via the polarizer or polarizers.
On account of its compactness and versatility, the wavelength-stabilized laser configuration according to the invention is particularly suitable for constructing high-rate laser modules having small structural sizes. Such a laser module contains, in addition to the wavelength-stabilized laser configuration, a hermetically sealed module housing accommodating the semiconductor laser, the optical isolator and at least the optical detectors and the optical filter of the regulating device. A laser module of this type has the advantage of an internal wavelength reference in conjunction with a compact and cost-effective mode of construction. It is advantageous, moreover, that, in the case of such a module, the requirements made of the filter are reduced further since the filter is exposed to no moisture at all and a constant operating temperature of the filter can also be brought about in a simple manner by coupling to a cooling device which cools the semiconductor laser.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a wavelength-stabilized laser configuration, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.